1. Technical Field
This invention is an apparatus for controlling flow into a compressor, and more particularly is a valve for controlling air intake when demand for service air from a compressor changes.
2. Background Art
A rotary screw air compressor has a pair of parallel rotors which are rotated oppositely relative to each other by a prime mover, such as an internal combustion engine or an electric motor. The rotors have intermeshed oppositely-pitched helical grooves. Operation of a compressor creates a negative pressure which draws air between the rotors and into the grooves. Air in the grooves is compressed when the intermeshed rotors rotate. Compressed air discharges from the grooves and is stored in a receiver tank. A manual service air valve selectively releases air from the tank for performing a task requiring application of air under pressure.
Pressure of air in the receiver tank increases when the service air valve is closed and air intake and rotor speed remain constant. Pressure of air in the receiver tank decreases when the service air valve is opened and air intake and rotor speed remain constant. Applicants' prior U.S. patent application Ser. No. 08/029,151 filed Mar. 10, 1993, now U.S. Pat. No. 5,388,967 and assigned to the assignee of this application, discloses a control and air inlet valve for controlling air intake and rotor speed in response to change in receiver tank pressure whereby air intake and rotor speed reduce when receiver tank pressure exceeds a pressure set-point and air intake and rotor speed increase when receiver tank pressure is less than the pressure set-point.
The inlet valve has a piston which controls movement of a valve member toward and away from a position in which the compressor air inlet is at least partially closed. A spring is located in a chamber on the one side of the piston and biases the valve member toward the closed position. Air from the receiver tank applies pressure against an opposite side of the piston and reduces bias on the valve member. Receiver tank pressure is admitted into the spring chamber when tank pressure exceeds the pressure set-point. Tank pressure in the chamber assists the spring bias the valve member toward the closed position and reduce air intake. Tank pressure is removed and pressure bleeds from the spring chamber to reduce bias on the valve member when tank pressure is less than the pressure set-point so that the valve member is permitted to move away from the closed position and increase air intake.
Response time of the control is a function of the time required to pressurize and de-pressurize the chamber, which, in turn, is a function of the air volume of the chamber. A difficulty with the applicants' above-discussed air inlet valve is that due to the volume of the chamber required to house the bias spring, the control can be unacceptably slow to respond to sudden changes in receiver tank pressure which occur when the service air valve is opened or closed.
Specifically, time required to increase pressure of air in the spring chamber to move the valve member toward the closed position after tank pressure exceeds the set-point results in a lag in the control. Time lag to close the inlet valve causes a spike in the system air pressure which can cause unwanted opening of a safety relief valve. Time required to bleed air from and decrease pressure of air in the spring chamber after tank pressure falls below the setpoint results in a lag before the piston reduces bias on the valve member and the valve member is permitted to move away from the closed position. Time lag to move the valve member away from the closed position causes a drop in the system air pressure.